The subject matter disclosed herein relates generally to combustors for gas turbine engines and more particularly to mixer assemblies for gas turbine engines.
Gas turbine engines, such as those used to power modern aircraft, to power sea vessels, to generate electrical power, and in industrial applications, include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. Generally, the compressor, combustor, and turbine are disposed about a central engine axis with the compressor disposed axially upstream or forward of the combustor and the turbine disposed axially downstream of the combustor. In operation of a gas turbine engine, fuel is injected into and combusted in the combustor with compressed air from the compressor thereby generating high-temperature combustion exhaust gases, which pass through the turbine and produce rotational shaft power. The shaft power is used to drive a compressor to provide air to the combustion process to generate the high energy gases. Additionally, the shaft power is used to, for example, drive a generator for producing electricity, or drive a fan to produce high momentum gases for producing thrust.
An exemplary combustor features an annular combustion chamber defined between a radially inboard liner and a radially outboard liner extending aft from a forward bulkhead wall. The radially outboard liner extends circumferentially about and is radially spaced from the inboard liner, with the combustion chamber extending fore to aft between the liners. A plurality of circumferentially distributed fuel injectors are mounted in the forward bulkhead wall and project into the forward end of the annular combustion chamber to supply the fuel to be combusted. Air swirlers proximate to the fuel injectors impart a swirl to inlet air entering the forward end of the combustion chamber at the bulkhead wall to provide rapid mixing of the fuel and inlet air.
Combustion of the hydrocarbon fuel in air in gas turbine engines inevitably produces emissions, such as oxides of nitrogen (NOx), carbon dioxide (CO2), carbon monoxide (CO), unburned hydrocarbons (UHC), and smoke, which are delivered into the atmosphere in the exhaust gases from the gas turbine engine. Regulations limiting these emissions have become more stringent. At the same time, the engine pressure ratio is getting higher and higher for increasing engine efficiency, lowering specific fuel consumption, and lowering carbon dioxide (CO2) emissions, resulting in significant challenges to designing combustors that still produce low emissions despite increased combustor inlet pressure, temperature, and fuel/air ratio. Due to the limitation of emission reduction potential for the rich burn-quick quench-lean burn (RQL) combustor, lean burn combustors, and in particular the piloted lean premixed/partially premixed pre-vaporized combustor (PLPP), have become used more frequently for further reduction of emissions. However, one of the major challenges for the development of PLPP is the requirement to sufficiently premix the injected fuel and combustion air in the main mixer of a mixer assembly within a given mixing time, which is required to be significantly shorter than the auto-ignition delay time.
Mixer assemblies for existing PLPP combustors typically include a pilot mixer surrounded by a main mixer with a fuel manifold provided between the two mixers to inject fuel radially into the cavity of the main mixer through fuel injection holes. The main mixer typically employs air swirlers proximate and upstream of the fuel injection holes to impart a swirl to the air entering the main mixer and to provide rapid mixing of the air and the fuel, which is injected perpendicularly into the cross flow of the air atomizing the fuel for mixing with the air. The level of atomization and mixing in this main mixer configuration is largely dependent upon the penetration of the fuel into the air, which in turn is dependent upon the ratio of the momentum of the fuel to the momentum of the air. As a result, the degree of atomization and mixing may vary greatly for different gas turbine engine operating conditions (e.g., low power conditions where there is poor atomization and mixing may result in higher emissions than high power conditions where there is better atomization and mixing). In addition, since the fuel injection holes are typically located downstream of the point where the air swirlers produce the maximum turbulence, the degree of atomization and mixing is not maximized, increasing the amount of emissions. Furthermore, since the fuel injection holes are typically located downstream of the air swirlers, the risk of flashback, flame holding and autoignition greatly increases due to the low velocity regions associated with fuel jets and walls. A highly possible source for flashback, flame holding and autoignition in the typical main mixer is caused by a wake region that can form downstream of the fuel injection holes where injected fuel that has not sufficiently penetrated into the cross flow of the air (e.g., when air is flowing at low velocity) will gather and potentially ignite. Another possible source is related to boundary layers along the wall, which is thickened by fuel jets due to reduced velocity.